unix/mpconfigport.h: set MICROPY_PY_SYS_PLATFORM to "darwin" if compiled on OSX

This change allows micropython to return the same value as CPython for sys.platform

unix/mpconfigport.h

@@ -73,7 +73,11 @@
 #define MICROPY_PY_ALL_SPECIAL_METHODS (1)
 #define MICROPY_PY_ARRAY_SLICE_ASSIGN (1)
 #define MICROPY_PY_SYS_EXIT         (1)
-#define MICROPY_PY_SYS_PLATFORM     "linux"
+#if defined(__APPLE__) && defined(__MACH__)
+    #define MICROPY_PY_SYS_PLATFORM  "darwin"
+#else
+    #define MICROPY_PY_SYS_PLATFORM  "linux"
+#endif
 #define MICROPY_PY_SYS_MAXSIZE      (1)
 #define MICROPY_PY_SYS_STDFILES     (1)
 #define MICROPY_PY_SYS_EXC_INFO     (1)